Uninformed punditry to the contrary, her comments violated no law or ethical rule. The judicial code of ethics says that judges are not to endorse or oppose candidates for elected office. These provisions, however, do not apply to Supreme Court justices. (Whether that exception is reasonable is a separate question.)
It’s true that, conventionally, justices steer clear of electoral politics. But that wasn’t always the case: In 1800, the members of the Supreme Court openly campaigned for the reelection of John Adams. This convention of silence, moreover, is inconsistent with one of the most basic underlying principles of the 1st Amendment: that more speech is better in a democracy because it leads to a better-informed population.